Methods for the extraction of uncompleted host RNA from turnip yellow mosaic virus infected Chinese cabbage plants and for the electrophoretic fractionation of ribosomes, ribosomal sub-units, polyribosomes and virus particles are described and characterised. The changes that occurred in ribosomal RNA fractions and in polyribosome status of sequentially ageing leaves of uninfected and TYMV-infected Chinese cabbage plants were examined using these techniques. The peak accumulation of chloroplast and cytoplasmic ribosomal RNA occurred at different times in the young leaves but following leaf maturity both fractions declined at a similar rate. The high levels of polyribosomes observed in the youngest leaves fell during leaf expansion to a level that was largely maintained after full leaf expansion was achieved. The effects of the virus on these patterns of change were attributable to its effect on chloroplast metabolism and periods of multiplication. The senescence of excised leaf discs was also studied and differences in the patterns of change of ribosomal RNA were found between different incubation treatments. Retardation of senescence of the leaf discs by lightor kinetin principally affected levels of cytoplastic ribosomal RNA. A different pattern of senescence in excised leaf discs and attached leaves was observed with an accelerated rate of loss of chloroplast ribosomalel RNA in the former. Kinetin was seen to elicit a different pattern of response in TYMV-infected discs compared to uninfected discs.